Toast last drink (7,2)
I believe the answer is:
bottoms up
'toast' is the definition.
(I've seen this before)
'last drink' is the wordplay.
'last' becomes 'bottom' (eg at the bottom of a list).
'drink' becomes 'sup' (to sup is to drink).
'bottom'+'sup'='BOTTOMS UP'
(Other definitions for bottoms up that I've seen before include "Your good health!" , "down it goes!" , "Toast" , "Down the hatch!" , "Cheers!" .)